A concave surface (referred to also as eye-ball side, or inner surface) of a spectacle lens is formed into a shape such as a spherical surface, a rotationally symmetric aspheric surface, a toric surface, a progressive surface, or a curved surface formed of a combination thereof, and when the surface shape is machined, for example, by cutting, it is mirror-polished to an optical surface. For mirror polishing a simple curved surface, such as a spherical surface or a toric surface, face-up grinding using a rigid abrasive platter, which is called Oscar polishing, is used.
The mirror polishing method using the abrasive platter is a method for transferring the surface shape of the abrasive platter to the polishing target. Therefore, a number of, for example, thousands of, types of machining platters corresponding to the number of surface shapes according to the lens prescriptions are necessary. Since the abrasive platter cannot be used for polishing complex surfaces other than these surfaces, so-called, free surfaces, such as a progressive surface, a resilient abrasive member is generally used.
For example, the present inventor proposes a polishing method in which polishing is performed while bringing part of a dome-shaped portion of the resilient abrasive member, which is selected from among a plurality of resilient abrasive members having dome-shaped portions, larger than the concave surface to be polished, of the polishing target and having different curvatures according to the surface shape of the surface to be polished, into abutment with substantially the entire surface of the surface to be polished. In other words, the curvature of the resilient abrasive member is properly selected depending on the shape of the workpiece. The polishing step includes polishing while applying pressure on the inner surface of the dome-shaped hollow resilient sheet with pressurized fluid to give a tension to the dome-shaped portion, swinging and rotating the polishing target about its own axis, and rotating the resilient polishing member until substantial alignment of the center of curvature of the dome-shaped portion with the center of the swinging motion of the polishing target is achieved.
The resilient abrasive member is selected, for example, by the steps of obtaining (Rmax+Rmax)/2=Rmid from the largest radius of curvature Rmax (inverse number of curvature) and the smallest radius of curvature Rmin existing on the inner surface of the lens, and selecting a resilient abrasive member having a dome-shaped portion having a radius of curvature close to the intermediate radius of curvature Rmid. In the case of the astigmatic surface (toric surface), the radius of curvature of the resilient abrasive member is an intermediate value between the base curve and the cross curve, and hence a toric surface having a cylindrical surface can be polished evenly with the resilient abrasive member which comes into hermetic contact therewith with good followability with a minimum degree of deformation thereof. However, for example, in the case of a spectacle lens for correcting strong astigmatism, the difference in curvature between the base curve and the cross curve is significant.
When an addition power, which is the difference between powers of near and distance portion of a progressive multi-focal lens, increases significantly, the difference in curvature between the distance portion and a near point increases significantly. Although the resilient abrasive member is superior in shape followability, in the case of the surface to be polished having a large difference in curvatures as described above, there may be a case in which some parts come into strong contact with such a surface and some parts come into poor contact with the same. Since the portion of poor contact cannot be easily polished, it requires a long time for polishing, while the portion of strong contact can be polished in a short time. Therefore, when an attempt is made to polish the portion of poor contact sufficiently, not only does it require a long polishing time, but it may result in so-called polishing sag, due to excessive polishing of the portion of strong contact. When trying to avoid polishing sag, the portion of poor contact may result in insufficient polishing.
Both polishing sag and insufficient polishing are defective polishing, and lowering of the yield and increasing the number of polishing steps due to the necessity of additional polishing have become problems. In view of such circumstances, it is an object of the present invention to provide a polishing method that can polish evenly a surface to be polished having a very large difference in curvatures evenly by the use of resilient polishing members.